GENERATION AND TRANSMISSION OF SIGNALS 43 



produced by the same structures that give rise to the induced waves. 

 The structural organization of the synaptic connections between the 

 primary olfactory fibres and the secondary neurons exhibits some interest- 

 ing features (Fig. 9). The presynaptic endings make contact exclusively 

 with the dendrites of the postsynaptic cells and not with their cell bodies. 

 The first synapse in the olfactory system therefore represents a purely axo- 

 dendritic interneuronal contact. Such a form of synaptic connection is 

 typical of the nervous system in invertebrates and in some lower vertebrates. 

 In higher vertebrates the majority of the synapses in the central nervous 

 system are axo-somatic, i.e. the presynaptic fibres make direct contact with 

 the cell bodies of the next neurons. This form of connection is considered 

 to provide possibilities for a more efiicient and rapid transmission of 

 impulses than the primitive axo-dendritic synapses, where the impulses 

 in the postsynaptic units are set up without any direct action on the cell 

 bodies. There seems further to be reason to believe that the transmission 

 in an axo-dendritic synapse occurs through a graded depolarization that 

 spreads electrotonically along the dendrites towards the cell body. This 

 implies that the signals are transmitted more slowly than in systems where 

 the impulses are passed from one unit to another by axo-somatic connec- 

 tions. The unique organization of the olfactory synapse as represented by 

 the densely interwoven nerve-nets of the glomeruli certainly also involves 

 other functional characteristics. It is most likely that the arborization of 

 the dendrites in the glomeruli has the function of collecting the incoming 

 impulses, thereby securing the transmission of the olfactory signals. It 

 is also possible that the glomeruli possess a functional specificity in the 

 sense that fibres from receptors with similar sensitivity properties are 

 directed towards particular glomeruli. 



REFERENCES 



Adrian, E. D. 1950. The electrical activity of the mammalian olfactory bulb. Electro- 



enceph. Clin. Neurophysiol. 2, 377-388. 

 Adrian, E. D. 1951. Olfactory discrimination. V Annee Psychol. 50, 107-113. 

 Arduini, a., Mancia, M. and Mechelse, K. 1957. Slow potential changes elicited in the 



cerebral cortex by sensory and reticular stimulation. Arch. Ital. Biol. 95, 127-138. 

 Beets, M. G. J. 1962. A molecular approach to olfaction. Molecular Pharmacology, 



ed. E. J. Ariens. Acad. Press. N.Y. 

 Coombs, J. S., Curtis, D. R. and Eccles, J. C. 1957. The interpretation of spike poten- 

 tials of motorneurones. J. Physiol. 139, 198-231. 

 Edwards, C. and Ottoson, D. 1958. The site of impulse initiation in a nerve cell of a 



crustacean stretch receptor. J. Physiol. 143, 138-148. 

 Gasser, H. S. 1956. Olfactory nerve fibres. /. Gen. Physiol. 39, 473-496. 

 Gehuchten, a. Van and Martin, I. 1891. Le bulbe olfactive chez quelques mammi- 



feres. La Cellule 7, 205-286. 

 Hartline, H. K., Wagner, H. G. and Ratliff, F. 1956. Inhibition in the eye of Limulus. 



/. Gen. Physiol 39, 651-673. 



